A glass-ceramic article is manufactured through the controlled heat treatment of a precursor glass body which causes the latter to be crystallized in situ. The production of the conventional glass-ceramic article comprises three general steps. First, a glass-forming batch, to which a nucleating agent is commonly added, is compounded and melted. Second, the resulting melt is simultaneously cooled rapidly enough to produce a crystal-free glass and an article of a desired geometry shaped therefrom. Third, the glass article is subjected to a particular heat treatment schedule to cause the glass to crystallize in situ. Customarily, this crystallization step is carried out in two parts. Thus, the glass article is initially heated to a temperature in the proximity of the annealing point thereof to develop nuclei in the glass and, thereafter, the temperature is raised (normally above the softening point of the glass) to cause the growth of crystals on the nuclei.
The crystallization in situ mechanism involves the essentially simultaneous growth of crystals on countless nuclei distributed throughout the glass body so that the microstructure of the normal glass-ceramic article consists of relatively uniformly-sized, fine-grained crystals homogeneously dispersed, but randomly oriented, within a residual glassy matrix, the crystal phase commonly constituting the predominant proportion of the article. Hence, glass-ceramic articles are customarily at least 50 percent and, frequently, are actually 75% by volume crystalline. This very high crystallinity commonly results in articles demonstrating chemical and physical properties quite different from those of the parent or precursor glass. Hence, the characteristics of a glass-ceramic article will normally approximate those of the crystal phase. Moreover and self-evidently, the residual glassy matrix will not only be small in amount but will also have a far different composition from that of the parent glass inasmuch as the components comprising the crystal phase will have been removed therefrom.
Since the manufacture of glass-ceramic articles is founded in the crystallization in situ of precursor glass bodies, conventional glass-forming fabrication processes such as blowing, casting, drawing, pressing, rolling, spinning, etc. can be utilized in shaping articles of desired configurations and dimensions. Furthermore, the presence of the residual glassy matrix insures that the glass-ceramic article is essentially free of voids and is non-porous.
For a more complete discussion of the theoretical concepts and practical considerations inherent to the production of glass-ceramic articles, reference is hereby made to U.S. Pat. No. 2,920,971, the basic patent in the field of glass-ceramics. That patent describes the glass-ceramic article as being predominantly crystalline, i.e., greater than 50% crystalline, and explains that the crystal phases developed in glass-ceramic articles are a function of the parent glass composition and the heat treatment parameters to which the parent glass is subjected.